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Abstract

Conscious perception depends on sensory evidence and on previous experience. The latter allows generating predictions, which take the form of a prior. The neural sources and mechanisms of this prior remain unknown. Here, we investigate prestimulus alpha oscillations as a candidate for the neural implementation of prior information. We recorded magnetoencephalographic activity in a task where we manipulated sensory evidence for and expectations of visually presented letters. Prestimulus alpha
power over left occipito-parietal sensors was higher when subjects could predict letter identity than when they could not. Source localization revealed an extended network including superior parietal, ventral occipital, superior temporal, and auditory cortices. Subsequent P1m/N1m also showed higher
amplitudes for predictable as compared to unpredictable stimuli, indexing the interaction of the prior with sensory evidence. This difference largely overlapped with the sources of prestimulus alpha activity, in particular primary auditory, superior temporal and superior parietal cortices. The involvement of auditory
and multisensory regions suggests that prestimulus alpha carries templates in the form of phonemes, which are compared against visual evidence, ultimately resulting in enhanced ERF amplitudes for matching expectations. In summary, we show that despite being based on visual information, top-down expectations were generated by networks involved in audiovisual integration.